These studies define the biochemical mechanisms whereby age-dependent changes in regulation of intermediary metabolism perturb physiological control systems and, thus, lead to a failure to maintain homeostasis in the aged. Investigations include: 1) regulation of pyruvate dehydrogenase activity by Ca2 ions; 2) alterations in the carnitine content of the heart with age; 3) preparation of isolated heart cells, and the study of carnitine transport by these cells; and 4) regulation of the metabolism of 25-hydroxyvitamin D3 in kidney mitochondria.